PURPOSE: To improve the radiation efficiency of radiating section thus to improve the radiation capacity of communication tube, by making the diameter of radiating section longer than the diameter of adiabatic section of communication tube while making the communication tube at the radiating section flat.

CONSTITUTION: Communication tube 13' is formed with a rising tube A15' for forming the adiabatic section 17', inclined section 14' for forming the radiating section 19' and a rising tube 16'. The inclined section 14' and the rising tube B16' are larger in diameter than the rising tube A15' and made flat. In other word, it has inner cross-section approximately same with that of the rising tube A15' while only the outer surface area is approximately 1.7 times that of the rising tube A15. Aluminum tape 18' is applied on the outer box 2 only at the radiating section to facilitate heat-exchange.